Optical fibre sensor assembly

ABSTRACT

An optical sensor assembly comprising a plurality of optical fibre sensor coils optically coupled by optical fibre; and an elongate support element, on which said plurality of optical fibre sensor coils and optically coupling optical fibre are mounted is disclosed. The support element has an elastic limit such that when said support element is bent from the elongate axis, the optical fibre fracture limit is reached before the elastic limit is reached. An array of these optical sensor assemblies, the mandrel on which the sensing coils are mounted and their method of manufacture are also disclosed.

The present invention relates to optical fibre sensor assemblies,optical sensing arrays, their method of manufacture, and mandrels formounting optical sensing coils and in particular, but not exclusively,to assemblies for optical hydrophones.

Optical hydrophone technology has been established now for approximately15 years. The use of fibre optics has many advantages—fibre is small andlightweight, immune to electro-magnetic interference (EMI), electricallypassive, capable of being used over long distances and it can be easilymultiplexed.

Optical hydrophones operate on the principle that pressure changescaused by an acoustic signal such as a sound wave are converted into astrain in a coil of optical fibre. This strain imposes a change in thephase of an optical signal passed through the coil, due to the physicalchange in length of the fibre and the stress optic effect. The phasechange can be detected by beating the signal with a reference signal ofa slightly different frequency which, when mixed, produces a beatfrequency, or heterodyne carrier, equal to the difference in frequencyof these two signals. The acoustic signal will therefore appear as aphase modulation on this carrier. It is known to form arrays of suchoptical hydrophones, which may be optically addressed using a variety ofmultiplexing techniques, e.g. time division multiplexing (TDM),wavelength division multiplexing (WDM), etc. Such hydrophone arrays arewell known and will therefore not be described. in detail herein. A moredetailed explanation of the addressing of such arrays may be found inPCT Application PCT/GB00/01300, Publication Number WO 00/62021 assignedto “The Secretary of State for Defence (GB)”.

To date both the US and the UK have successfully demonstrated opticalfibre hydrophones for use in many underwater sensing applications andthese have been shown to exhibit performances on a par withpiezo-electric sensors—typically achieving Deep Sea State Zero (DSS0)noise performance up to in excess of 5 kHz.

Although optical fibre hydrophones have many advantages, optical fibresthemselves are inherently fragile. Hydrophones are often used in harshenvironments, for example in towed arrays. In these situations the mostimportant factor that needs to be addressed is the mechanicalsurvivability of the array of hydrophones. This is because of thedeployed environment and the harsh way in which they aretreated/handled. When deployed the array will see huge amounts of energyin the form of vibration transmitted to it down the tow cable andturbulence from the fact that it is travelling through the water. Thehandling consists of winching, de-tensioning and reeling plus thepossibility of it laying around on the deck of a ship with the risk ofit being kicked and trodden on. Add to this the enormous temperaturerange that the array will have to see, direct sunlight on the equatorand the long, cold arctic nights, and you have a lot of danger to theintegrity of the array.

Secondly there are spurious signals imparted onto the sensor by thenoise sources within the array such as vibration and noise caused by thearray passing through the water (flow noise).

All of the above dangers and noise sources need to be reduced throughthe mechanical design of the array. This is no mean feat when youconsider how delicate optical fibre can be.

FIG. 1A shows a cross section of a known fibre optic hydrophone, andFIG. 1B shows a cross section of a known “air backed” fibre optichydrophone. Air backed hydrophones comprise a fibre optic sensing coil10 mounted on a mandrel comprising an inner rigid support member 5 and aflexible former 7. They have generally been formed in two parts of twodifferent materials, so that an inner rigid member to give the mandrelrigidity could be used along with a more flexible former for mountingthe optical sensing coil on. In order to increase sensitivity of thehydrophone to acoustic vibrations, there is an air gap 19 between theflexible former and its rigid support member. Conventionally this airgap is sealed by adhesives or by an interference fit between the formerand the inner rigid support member.

A problem with this device when used in environments having changingexternal pressures and temperatures, such as in towed arrays, is thatthe flexible former, rigid support member and any adhesive between themtend to contract at different rates and this along with the fact thatadhesives are often brittle can lead to the air gap not remainingpressure tight, which affects the readings of the hydrophone.

In addition to the use of individual hydrophones, it is known to usegroup hydrophones (see FIG. 3) and spatially extended hydrophones (seeFIG. 4), these comprise an extended length hydrophone coil, connectedeither singly or in groups, to form a continuous acoustic aperture. Theextended hydrophone length could be a maximum equal to the group length,but in practice an extended hydrophone group will probably comprise fourcoils.

The use of a plurality of optical fibre sensing coils making up a singlehydrophone has the disadvantage that the fibre between the coils issensitive to any kind of strain. This strain would be indistinguishablefrom a real acoustic signal on a single hydrophone. Also the twisting ofthe hydrophones relative to each other can snap the fibre at the pointat which it exits the hydrophone.

U.S. Pat. No. 6,118,733 discloses an “interlink” for linking spatiallyextended hydrophones. The interlink links two hydrophones and has ahelical spring like form, with the optical fibre linking the hydrophonespassing along a helical groove on the interlink. A hydrophone isattached to either end of the interlink, typically by attaching themandrel of the hydrophones to the interlink by epoxy. The optical fibreis held immobile on the flexible interlink and thus is protected fromexternal forces. This design protects the optical fibre betweenhydrophones. It does, however, have the disadvantage of providing noprotection to the hydrophone itself. Furthermore, a separate interlinkneeds to be attached for each additional hydrophone, which is fairlytime consuming and expensive.

Typically, conventional piezo-electric hydrophones in towed arrays areconnected together by cords.

According to a first aspect of the present invention, there is providedan optical sensor assembly comprising: a plurality of optical fibresensor coils optically coupled by optical fibre; and an elongate supportelement, on which said plurality of optical fibre sensor coils andoptically coupling optical fibre are mounted; wherein said supportelement has an elastic limit such that when said support element is bentaway from the elongate axis, the optical fibre fracture limit is reachedbefore the elastic limit is reached.

Connecting together a plurality of optical fibre sensor coils, has theadvantage of allowing their construction to be performed in a singleprocess, this is not only more efficient than a multi stage process butit also results in lower optical losses between the coils. However, thissystem does have an inherent problem in that the optical fibre couplingthe sensing coils is particularly sensitive to external strains, whichnot only endangers the structural integrity of the system, but may alsoaffect any optical signal being transmitted by the optical fibre. Thisproblem is addressed by mounting the coils and the coupling opticalfibre on a single elongate support element, which acts to protects theoptical fibre of the coil and the optical fibre connecting the coilsfrom stress and strain. The elongate support element needs to be anelastic material such that it can be bent but will regain its elongatenature when the force is removed. The elastic limit of the supportelement is chosen such that when it is bent away from the elongate axis,the fracture limit of the optical fibre that it is supporting is reachedbefore the elastic limit is reached. Thus, the optical fibre willfracture before the elastic limit of the support element is exceeded.This ensures, that provided the support element is not bent so much thatthe optical fibres mounted thereon will break, it will be within itselastic limit and thus, on removal of the bending force willsubstantially regain its elongate shape.

Advantageously, said support element has an elastic limit such that whensaid support element is bent away from the elongate axis around a curvehaving a radius of half a metre the support element elastic limit is notreached. In some embodiments of the invention, such as where the opticalsensor assembly is a part of a towed array hydrophone, the supportelement needs to be flexible enough to be able to bend around a drum onwhich towed array hydrophones are generally mounted on board ship, butrigid enough to regain its shape when removed from the drum. Thediameter of these drums is typically in the region of a metre.

Although the support element can take a plurality of different forms inpreferred embodiments it comprises a flexible rod. A flexible rod may bea cheap and easy to handle structure, upon which it is simple to mountoptical fibres and optical fibre sensing coils.

In preferred embodiments, said support element comprises a carbon fibrerod. The properties of carbon fibre have been found to be particularlysuitable for this purpose. Furthermore, it is fairly cheap, robust andeasy to handle.

In other embodiments said support element comprises a steel rod.

Although said plurality of optical fibre sensor coils may be arranged ina plurality of different configurations in preferred embodiments theyare arranged optically in series with each other.

In preferred embodiments, said plurality of optical fibre sensor coilsare mounted on said support element such that the distance betweenadjacent coils is substantially identical. The separation between thecentre of the groups of sensing coils affects the frequencyinterrogated. By arranging the sensing coils on a support element whichis substantially incompressible the distance between the coils issubstantially invariant. this means that the centre of the group isfixed and it is thus, simpler to maintain a constant distance betweenthe centres of different groups. The distance between group centres setsthe upper interrogation frequency limit.

Preferably, said assembly further comprises a plurality of hollowmandrels corresponding to said plurality of optical fibre sensor coils,each of said mandrels having an internal and an external surface;wherein each of said plurality of optical fibre sensor coils is woundaround said external surface of said corresponding mandrel, saidplurality of optical fibre sensor coils being mounted on said supportelement by connecting a portion of said internal surface of saidcorresponding mandrel to said support element. Connecting the mandrel tothe support element is a simple and effective way of mounting thehydrophone. The connection can be made directly by, for example, gluingthe mandrel to the support element, or it may be made via connectionmeans.

More preferably, said portion of said internal surface connected to saidsupport element comprises a central portion of said internal surfacesubstantially mid way between either end of said mandrel. The attachmentof the central portion of the mandrel to the support element provides asymmetrical mounting arrangement and makes it less susceptible tovibrations in other parts of the assembly.

Although the mandrel could take a variety of forms, preferably it iscylindrical in shape.

Preferably, said mandrel comprises an inner member and an outer former;wherein said outer former is mounted via at least one compressible sealon said inner member such that there is a cavity between said outerformer and said inner member, said compressible seal being significantlymore compressible than said outer former or said inner member such thatit is operable to seal said cavity across a range of temperatures andpressures. The use of a compressible seal enables the cavity to remainair tight across a range of temperature and pressures and thus, improvesthe accuracy of the device particularly in harsh environments such as intowed arrays.

In preferred embodiments, at least one portion of said support elementcomprises an external surface that is compressible. A compressiblesurface allows the coupling optical fibre mounted thereon to bedecoupled from any vibrations travelling down the support element.Furthermore, it allows the optical fibre to be mounted on the supportelement under tension such that the material is compressed but not fullycompressed. This eases the mounting procedure and also reduces anystrain transfer from the support element to the optical fibre.

A simple way of producing an external compressible surface is to mount acompressible material to cover said at least one portion of said supportelement.

Preferably said at least one portion of said support element comprisinga compressible external surface includes said portions of said supportelement between said plurality of optical fibre sensor coils. It isthese portions on which said coupling optical fibre is mounted, and asit is this fibre that is particularly vulnerable and needs the extraprotection of mounting on a compressible surface.

Preferably, said coupling optical fibre is wound around saidcompressible external surface of said support element. This arrangementprovides a robust means of mounting the optical fibre. Winding the fibretoo tightly on a non-compressible surface may lead to strain inducedbreakages. The use of a compressible surface enables the fibre to bewound upon the surface with some degree of give in the arrangement.

Advantageously, said assembly further comprises compressible materialcovering said optically coupling optical fibre and said compressibleexternal surface. This compressible material provides protection for thecoupling optical fibres.

Preferably there is a sheath of compressible material covering an outerenvelope of said optical sensor assembly. This provides additionalprotection of the system and also protects the sensors from extraneousnoise.

In preferred embodiments, said compressible material comprises opencelled foam. Although it is possible to use closed celled foam, opencelled is preferred as it can absorb fluids, such as buoyant fluids.Foam is a cheap, robust and readily available compressible material.

Preferably, said assembly further comprises a protective cover mountedto substantially surround an outer surface of each of said plurality ofoptical fibre sensing coils. More preferably the protective cover ismounted by attachment to said support element.

Covers are advantageous in that they impede objects from rubbing on thesurface of the sensor coils and contributing to the noise signal. Thecovers also protect the sensor coils from external loads. In order toprevent vibrations from being transferred to the sensor coils the covershave to be mounted such that they don't come into contact with thesensor coils in any way. A simple way of doing this is to mount thecovers on the support element.

Although the optical sensor assembly may comprise any number of opticalfibre sensing coils, it has been found to be particularly effective touse four optical fibre sensor coils.

Although the plurality of optical fibre sensing coils, may each comprisea single hydrophone (an arrangement such as is shown in FIG. 2) or theymay comprise an extended aperture hydrophone (as is shown in FIG. 4),preferably said plurality of optical sensing coils comprise a singlehydrophone. Generally, this would be in an arrangement as is shown inFIG. 3.

According to a second aspect of the present invention there is providedan optical sensing array comprising a plurality of optical sensorassemblies according to a first aspect of the present invention, saidplurality of optical sensor assemblies being in optical communicationwith each other.

Although the plurality of optical sensor assemblies can be mechanicallyattached to one another by any means, a particularly cheap and effectiveway of doing it is to use cords.

Preferably, said sheath of compressible material covers said opticalsensing array. By covering the whole array with compressible material, adegree of mechanical protection is provided to the array, along with acertain amount of acoustic insulation.

Advantageously, said array further comprises a protective cover, saidarray having a substantially cylindrical outer envelope and saidprotective cover being in the form of a hose. Mounting the whole arraywithin a hose provides a degree of mechanical protection, and yet stillprovides some flexibility, the flexibility is required for themanipulation, such as mounting on drums typically having diameters ofthe order of a metre, the array.

In preferred embodiments said array further comprises a buoyant fluidconfined within said hose. The hose also provides a containment vesselfor a buoyant fluid, which can be introduced into the hose to provideneutral buoyancy for the device in water. The use of open celled foam asthe compressible material allows the introduction of a substantialquantity of buoyant fluid such as kerosene.

According to a third aspect of the present invention, there is provideda mandrel for supporting an optical sensing coil, said mandrelcomprising: an inner member; an outer former; and at least onecompressible seal; wherein said outer former is mounted via said atleast one compressible seal on said inner member such that there is acavity between said outer former and said inner member, saidcompressible seal being significantly more compressible than said outerformer or said inner member such that it is operable to seal said cavityacross a range of temperatures and pressures.

In air-backed hydrophones of the prior art such as the hydrophoneillustrated in FIG. 1B, the rigid inner member and flexible former havetraditionally been joined by adhesive or by an interference fit.Traditionally, there has been a desire to provide a rigid seal betweenthe two, this has been in line with the desire to provide a symmetricalhydrophone, a symmetrical arrangement being more resistant to extraneousinfluences. This desire for a rigid seal has led to the former beingjoined to the inner member by an interference fit or by an adhesive thathas a compressibility that is similar to that of the former. A problemwith rigid seals has been that in environments where external pressureand temperature are subject to change, such as in the towed arrayenvironment, the different rates of expansion and contraction of theformer, inner support member and adhesive have caused the rigid seal ofthe air cavity to fail. The present invention addresses these problemsby using a compressible seal, which has a compressibility which issignificantly greater than that of the former of the inner member. Thisenables the seal to maintain its integrity across a range of temperatureand pressure variations. In most embodiments the hydrophones arerelatively small and any perceived problems due to lack of symmetry fromthe compressible seals have not emerged.

Although it is possible to seal the cavity with a single suitably shapedseal, it is generally preferable to use two compressible seals, one ateither end of the cavity, the sidewalls of the cavity being formed bythe inner member and former.

Advantageously said two compressible seals comprise O-rings. O-rings area simple and effective way of sealing the cavity. Although the O-ringsare smaller than traditional O-rings, preferably, having an externaldiameter of between 5 and 30 mm, they have been found to be veryeffective as seals.

In preferred embodiments said at least one compressible seal has athickness of between 1 and 5 mm. This thickness of seal has been foundto provide sufficient flexibility to maintain a good seal across a widerange of temperature and pressure variations.

Although the seal can be made from a variety of compressible andflexible materials, rubber has been found to be a particularly effectivematerial for the seal.

Preferably, in order to hold the seal in place effectively, said innermember comprises two recesses, said two compressible seals being mountedwithin said two recesses.

In preferred embodiments said inner member is made of a substantiallyrigid material, preferably of metal.

Advantageously, said former is made of a flexible material, preferablyof plastic. Plastic has been found to be particularly suitable materialfor making a former, it is cheap and easy to manufacture and it has thenecessary flexibility and durability required.

According to a fourth aspect of the present invention there is provideda method of constructing an optical sensor assembly, comprising a firststep of winding from a single piece of optical fibre, a plurality ofoptical fibre sensor coils with optically coupling optical fibretherebetween onto a support element; said support element having anelastic limit such that when said support element is bent away from theelongate axis, the optical fibre fracture limit is reached before theelastic limit is reached.

Constructing an optical sensor assembly comprising several sensors froma single piece of optical fibre has the advantages of ease ofmanufacture and low optical losses. Furthermore, mounting the sensorsand optical coupling fibres on a support element provides mechanicalstability to the device, which provides a degree of protection to thefibres from external stresses and strains which may not only causedamage to fibres, but also affect any optical signal passing along thefibre. It also aids in the construction, winding onto a single elongatesupport element being a fairly simple way of constructing the device.

Preferably, a plurality of hollow mandrels corresponding to saidplurality of optical fibre sensor coils are mounted on said supportelement, said optical fibre sensor coils being wound onto said mandrels.Winding directly onto the support element or mandrels mounted on saidsupport element, allows the device to be produced in one singleefficient step.

Advantageously, said at least one portion of said support elementincludes said portions of said support element between said plurality ofoptical fibre sensor coils, said portion of said optical fibre opticallycoupling said optical fibre sensor coils being wound onto saidcompressible material under tension such that said compressible materialis not totally compressed. In order to prevent the optical fibre frombreaking when the support element about which it is wound bends, theoptical fibre should be wound loosely. Given the extremely “springy”nature of the fibre this can be a difficult task. The use of acompressible material significantly eases the task, the fibre beingwound under tension onto the compressible material thereby alleviatingthe problems due to the springiness of the fibre, while the fact thatthe material is not totally compressed gives the fibre some protectionfrom the bending of the rod.

According to a further aspect of the present invention there is provideda method of constructing an optical sensing array comprisingconstructing a plurality of optical sensor assemblies according to afourth aspect of the present invention, said method further comprisingthe steps of: linking said optical sensor assemblies together to formsaid sensor array; said linking being done mechanically by cord, andoptically by optical fibres, said linking optical fibres being longerthan said cord.

The compressible material mentioned in this application is in preferredembodiments foam and in more preferred embodiments open celled foam.However, any material that is sufficiently compressible such that anoptical fibre being wound on the material under tension is sufficient tocause compression of the material would be suitable. The tension ofwinding is chosen such that the material is not fully compressed andthus, any strain transfer between the support element and the opticalfibre is reduced.

Particular embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, ofwhich:

FIG. 1A shows a schematic representation of a fibre optic hydrophone;

FIG. 1B shows a cross section of a conventional air-backed fibre optichydrophone;

FIG. 2 shows a schematic representation of a plurality of pointhydrophones mounted in series;

FIG. 3 shows a group hydrophone configuration;

FIG. 4 shows an extended aperture hydrophone configuration;

FIG. 5 shows an optical sensing assembly according to an embodiment ofthe present invention;

FIG. 6A shows a portion of the sensing assembly of FIG. 5 in moredetail;

FIG. 6B shows an expanded view of the mandrel of FIG. 6A;

FIG. 7 shows a cross section of the protective cover support elements ofFIG. 6A;

FIG. 8 shows a sensing coil mounted on a winding machine duringfabrication;

FIG. 9 shows an optical sensing array according to an embodiment of thepresent invention; and

FIG. 10 shows a towed array hydrophone.

Referring to FIG. 1A, there is shown a known optical hydrophone assembly1. The hydrophone assembly 1 comprises a tubular mandrel with a former 7mounted about an inner member 5. A hydrophone coil 10 comprising a coilof optical fibre is coiled around the former. In the assembly shown inFIG. 1 the coil 10 is represented schematically, and would in generalconsist of several layers of tightly wound optical fibre. In addition,before use, the hydrophone assembly may require waterproofing to protectthe hydrophone elements, including elements internal to the hydrophonenot shown in FIG. 1, from water damage.

It is known to use a plurality of hydrophones, such as that illustratedin FIG. 1, connected in series, as shown in FIG. 2. The hydrophones havedimensions comparable to the existing Benthos AQ4 hydrophone. Thisconfiguration requires one optical reflector for each coil.

An alternative arrangement, and one used in embodiments of thisinvention is shown in FIG. 3. Here a group of point hydrophones areconnected in series to form a single array channel, with four pointhydrophones. This configuration requires one optical reflector betweenevery group of four hydrophones.

FIG. 4 shows an alternative arrangement comprising an extended aperturehydrophone. Extended aperture hydrophones may suffer from excessiveacceleration sensitivity. The group hydrophone is a practical way ofcreating an extended hydrophone that alleviates this problem.

FIG. 5 shows an optical sensing assembly 30 according to an embodimentof the present invention. The sensing assembly comprises a plurality offibre optic sensing coils 10, generally four, mounted on individualmandrels 12. The mandrels 12 are mounted on a flexible carbon fibre rod14. Individual sensing coils 10 are coupled together by optical fibres16. These fibres 16 are mounted on the flexible rod 12 by winding onto acompressible material 18 coating the section of the rod between themandrels 12.

FIG. 6A shows a portion of the sensing assembly of FIG. 5 in moredetail. It shows a mandrel 12 is attached to the rod 14 via a mountingpoint 15 located at its centre point. Mounting the mandrel 12 centrallyprovides a symmetrical arrangement and helps to isolate the mandrel 12from vibrations passing along the rod 14. Each mandrel has a protectivecover 20 mounted to cover the mandrels and protect the sensing coil 10,this protective cover is mounted on the rod via protective cover supportelements 22. The rod thereby supports covers for the coils. These coversare needed to stop any external objects from rubbing on the surface ofthe coils and contributing to the noise signal. The covers also protectthe coils from crushing loads. In order to inhibit vibrations from beingtransferred to the sensing coils, the covers have to be mounted suchthat they don't come into contact with the coils in any way. The coversare thus mounted on the central rod.

Foam 24 is used to cover the coupling fibres 16 (see FIGS. 5 and 6A) andthereby reduce flow noise and bulge waves that pass through a fluidfilled array. This also protects the fibre from catching on any externalstrain members.

FIG. 6B shows an expanded view of the mandrel 12 of FIG. 6A. The mandrel12 is formed of a former 13 and an inner support member 15. An O-ring 17is located between the former 13 and inner support member 15, thisO-ring acts to seal the air gap 19 that there is between the former 13and inner member 15. The inner member 15 is formed of a substantiallyrigid material such as a metal, whereas the former 13 is formed of amaterial that has some flexibility, generally a plastic. Typically theinner member 15 is formed from aluminium or titanium, while the former13 may be made of ABS, PVS or Delryn™. The inner member 15 provides themandrel with the required stiffness and strength while the former 13provides a flexible mounting for the optical sensing coil 10. The airgap increases the sensitivity of the hydrophone. The O-ring is generallymade of rubber and is typically between 5 and 30 mm in diameter andbetween 1 and 5 mm thick.

FIG. 7 shows a cross section of the protective cover support elements22, with a cut 22 a through which the optical fibre passes from themandrel 12 to the foam 18 surrounding the rod.

FIG. 8 shows. a mandrel 12 mounted on a winding machine duringfabrication. During fabrication the protective cover support elements 22are used as steadies for the winding machine. The four optical sensingcoils 10 and their connecting optical fibre 16 (see FIG. 5 and 6B) arewound in one go from a single piece of optical fibre. When the fibre istraversing between the single sensing coils it is coiled around thecentral rod. Under reeling the rod bends thus the fibre has to beloosely wound around the rod to avoid strain-induced breakage. The taskof winding fibre loosely and in a controlled manner around a rod is adifficult task as fibre is extremely springy with it wanting to becomestraight. To get around this problem the fibre is wound under tensiononto a foam such that the foam isn't totally compressed. This allows thefibre some degree of freedom to cope with any strain while still keepingit under held in position (see FIG. 5).

FIG. 9 schematically shows a portion of an optical sensing arrayaccording to an embodiment of the present invention. The optical sensingarray 40 comprises a plurality of optical sensing assemblies 30 asillustrated in FIGS. 5 and 6 optically coupled together by optical fibre32. They are further mechanically coupled using cords 34. There aregenerally two cords coupling the optical sensing assemblies, withoptical fibres passing alongside them, the optical fibres being longerthan the cords. A sheath of open celled foam 36 covers the wholeassembly providing a degree of both mechanical and acoustic protection.The whole is then mounted within a waterproof hose 38 like construction.This may be in the form of a polyurethane casting. The hose constructionoffers further mechanical protection whilst allowing the arrayflexibility so that it can be wound around drums for storage and toallow it to be dispensed and retrieved from its sensing environment.

A waterproof hose is needed to protect the array from water as inpreferred embodiments it is intended for underwater use. This use may bein saline water and saline water rapidly degenerates optical fibres,thus a waterproof covering is required. A further advantage of thewaterproof hose is that may also be used to contain a fluid. Thus, afluid can be introduced into the hose and can lie within the open cellsof the foam and thereby make the device neutrally buoyant. A typicalfluid to be used is kerosene.

FIG. 10 shows a towed array hydrophone comprising an optical sensingarray 40 according to an embodiment of the invention. The towed arrayhydrophone is mounted around a drum 42. The support elements for theoptical sensing coils or hydrophones are flexible enough to bend aroundthis drum. The elastic limit of the support elements is not reached whenthey are wound around the drum, thus, when the array is unwound from thedrum the support elements and thus the array itself resumes itssubstantially elongate nature. The drum is typically of the order of ametre in diameter, although drums varying in diameter from 0.5 metre to2 metres may be used.

The type of material that the support element is made from may beselected according to its proposed use, and factors such as the size ofthe drum it may be wound around. Furthermore, the nature of the opticalfibre is selected according to the mechanical stresses it is to besubjected to and also the optical properties required. Typically theoptical fibre has a diameter of 180 μm, while the support element ismade of carbon fibre having a 2 mm diameter.

1. An optical hydrophone assembly comprising: a plurality of opticalfibre hydrophone sensor coils, responsive to imposed strain to produce achange in phase of an optical signal passing therethrough, saidhydrophone sensor coils being longitudinally spaced optically coupled byoptical fibre; and an elongate support element, on which said pluralityof optical fibre sensor coils and optically coupling optical fibre aremounted; wherein said support element has an elastic limit such thatwhen said support element is bent away from the elongate axis, theoptical fibre fracture limit is reached before the elastic limit isreached.
 2. An optical sensor assembly according to claim 1, saidsupport element has an elastic limit such that when said support elementis bent away from the elongate axis around a curve having a radius ofhalf a metre the support element elastic limit is not reached.
 3. Anoptical sensor assembly according to claim 1, wherein said supportelement is a flexible rod.
 4. An optical sensor assembly according toclaim 3, wherein said rod has a circular cross section.
 5. An opticalsensor assembly according to claim 3, wherein said support elementcomprises a carbon fibre rod.
 6. An optical sensor assembly according toclaim 3, wherein said support element comprises a steel rod.
 7. Anoptical sensor assembly according to claim 1, wherein said plurality ofoptical fibre sensor coils are arranged optically in series with eachother.
 8. An optical sensor assembly according to claim 1, wherein saidplurality of optical fibre sensor coils are mounted on said supportelement such that the distance between adjacent coils is substantiallyidentical.
 9. An optical sensor assembly according to claim 1, saidassembly further comprising a plurality of hollow mandrels correspondingto said plurality of optical fibre sensor coils, each of said mandrelshaving an internal and an external surface; wherein each of saidplurality of optical fibre sensor coils is wound around said externalsurface of said corresponding mandrel, said plurality of optical fibresensor coils being mounted on said support element by connecting aportion of said internal surface of said corresponding mandrel to saidsupport element.
 10. An optical sensor assembly according to claim 9,wherein said portion of said internal surface connected to said supportelement comprises a central portion of said internal surfacesubstantially mid way between either end of said mandrel.
 11. An opticalsensor assembly according to claim 9, wherein said mandrel issubstantially cylindrical in shape.
 12. An optical sensor assemblyaccording to claim 9, wherein said mandrel comprises an inner member andan outer former; wherein said outer former is mounted via at least onecompressible seal on said inner member such that there is a cavitybetween said outer former and said inner member, said compressible sealbeing significantly more compressible than said outer former or saidinner member such that it is operable to seal said cavity across a rangeof temperatures and pressures.
 13. An optical sensor assembly accordingto claim 1, at least one portion of said support element comprising anexternal surface that is compressible.
 14. An optical sensor assemblyaccording to claim 13, said compressible external surface of saidsupport element comprising a compressible material mounted to cover saidat least one portion of said support element.
 15. An optical sensorassembly according to claim 13, wherein said at least one portion ofsaid support element comprising a compressible external surface includessaid portions of said support element between said plurality of opticalfibre sensor coils.
 16. An optical sensor assembly according to claim15, wherein said optically coupling optical fibres are wound around saidcompressible external surface of said support element.
 17. An opticalsensor assembly according to claim 16, said assembly further comprisingfurther compressible material covering said optically coupling opticalfibre and said compressible external surface.
 18. An optical sensorassembly according to claim 1, further comprising a sheath ofcompressible material covering an outer envelope of said optical sensorassembly.
 19. An optical sensor assembly according to claim 14, whereinsaid compressible material comprises open celled foam.
 20. An opticalsensor assembly according to claim 1, said assembly further comprising aprotective cover mounted to substantially surround an outer surface ofeach of said plurality of optical fibre sensing coils.
 21. An opticalsensor assembly according to claim 20, said protective cover beingmounted by attachment to said support element.
 22. An optical sensorassembly according to claim 1, said optical sensor assembly comprisingfour optical fibre sensor coils.
 23. An optical sensor assemblyaccording to claim 1, wherein said plurality of optical fibre hydrophonesensor coils form a single hydrophone.
 24. An optical sensing arraycomprising a plurality of optical sensor assemblies as claimed in claim1, said plurality of optical sensor assemblies being in opticalcommunication with each other.
 25. An optical sensing array according toclaim 24, wherein said plurality of optical sensor assemblies aremechanically attached to one another by cords.
 26. An optical sensingarray according to claim 24, said array further comprising a protectivecover, said array having a substantially cylindrical outer envelope andsaid protective cover being in the form of a hose.
 27. An opticalsensing array according to claim 26 said array further comprising abuoyant fluid confined within said hose.
 28. An optical sensing arrayaccording to claim 27, wherein said buoyant fluid is kerosene.